While the electromagnetic aspects of hairpin windings are actively being investigated and discussed in recent literature, including the design rules together with the loss calculation and reduction techniques, the thermal performance and modeling aspects have received less attention to date. In hairpin windings, the conductors (pins) are comparatively larger and arranged as separate components in parallel within the slot. In contrast, conductors randomly overlap and contact each other for traditional random windings. The differences in the aforementioned winding physical characteristics result in a different methodology to develop the thermal network. This article presents a 3-D lumped parameter thermal network (LPTN) approach for an oil-spray cooled hairpin winding, which includes the slot thermal model configuration, the end-winding connections, together with different methodologies of analyzing the end-winding sprayed oil characteristics. The aforesaid thermal model captures unique features related to the winding technology and cooling mechanism, such as the nonuniform end-winding temperature caused by the uneven oil-spray cooling effects. Finally, taking an existing propulsion drive hairpin stator and a bespoke-designed test setup, the presented steady-state thermal modeling approach is experimentally validated covering various experimental tests, including different spray conditions.

This paper proposes a novel, low-cost, effective way to improve the thermal performance of electrical machines by extending a part of the back-iron into the slot. This modification helps in reducing the thermal resistance path from the center of the slot to the coolant, however its thermal benefits must be clearly evaluated in conjunction with the electromagnetic aspects, due to the higher iron losses and flux-leakage, and furthermore such an extension occupies space which would otherwise be allocated to the copper itself. Taking a case study involving an existing 75kW electric vehicle (EV) traction motor, the tradeoffs involving the losses, fluxleakage, output torque, torque-quality and the peak winding temperature with back-iron extension (BIE) and without are compared. Finally, experimental segments of the aforesaid motor are tested, verifying a significant 26.7% peak winding temperature reduction for the same output power with the proposed modification.

Thermal management is often considered a bottleneck in the pursuit of the next-generation electrical machines for electrified transportation with a step change in power density. Slot-channel cooling is considered to be an effective cooling technique, either as an independent method or as a secondary heat transfer path, which compliments traditional cooling systems. The slot-channel specific geometry and position effects on the thermal benefits are not thoroughly investigated in the literature, while previous work focuses on passing fluid through the unused space left in between coils forming concentrated windings. In this article, slot-channel cooling is implemented within an oil-flooded cooling system for a high power density motor that is used as a pump. A flexible and detailed lumped parameter thermal network (LPTN) is proposed for the cooling system, with the LPTN used to optimize the slot-channel dimensions and location for obtaining maximum thermal benefits. Finally, a surface-mount permanent magnet (SPM) machine with the optimized slot channel geometry is built and tested to validate the thermal model, experimentally achieving an armature continuous current density in excess of 30 A/mm ² .

Oil jet cooling and hairpin windings have become popular in electrical machine design for modern electric vehicles. In recent years, these two technologies are often combined and can be found in many commercial products. The highly effective cooling capability of oil jets and the high thermal conductance of hairpin windings greatly benefit the machine performance metrics. However, published quantitative literature on this topic is relatively scarce. This article presents experimental results and practical insights of oil jet cooling on the end region of hairpin windings. Temperature distribution, heat transfer coefficient, and cooling efficiency are all investigated. Consistent temperature profiles are observed throughout the range of flow rates. Moreover, the effects of using different types of oil and inclination angles are researched as well. Analysis of variance indicates that these two factors affect various aspects of the oil jet cooling’s performance. Results are also compared against data obtained from previous studies on oil spray cooling. This study can help engineers implement oil jet cooling in machine designs with hairpin windings.

The rapid development of transportation electrification brings unprecedented opportunities and challenges as well to the technological improvement of high-power density motors. As the major restriction of the motor performance, thermal issues attract more attentions recently and the heat pipe (HP), due to its high thermal conductivity and lightweight characteristics, is identified as a promising solution. Though HPs have been attempted in cooling systems of electrical machines, their thermal benefits still need to be fully understood. This article provides a comprehensive review and performs a series of experimental investigations to fill the technology gaps for the HP application in electrical machines. Firstly, the HP technology is briefly introduced, followed by a detailed review on its applications in different locations of electrical machines with corresponding thermal improvements reported. Then, experimental tests are conducted to investigate the correlation between the HPs’ parameters and their cooling effectiveness, including cross-sectional shape, bending angle, fins arrangement and inclination angle. The obtained results and conclusions could provide a guideline for the promotion of HPs in next generation electrical machines for future transportation.

The present paper deals with a review on bearing currents in electrical machines, with major emphasis on mechanisms, impacts, and mitigation strategies. High-frequency common-mode voltages from the inverter-driven system have been found to be the main reason for bearing current leading to motor bearing degradation and eventual failure. This paper deals with bearing currents—electrical discharge machining (EDM) currents, circulating bearing currents, and rotor-to-ground bearing currents—and the various methods of their generation and effects that are harmful to the bearings and lubricants of a motor. Mitigation techniques, among which the following have been taken into account, are studied in this context: the optimization of PWM modulation, and the use of shaft grounding brushes, insulated bearings, and passive or active filters. Finally, advantages, limitations, and implementation challenges are discussed. A review comparing three-phase and dual three-phase inverters showed that, due to the increased degree of freedom in modulation strategies, it is possible to eliminate common-mode voltages through active modulation techniques. Such added flexibility will reduce the risk of bearing currents effectively. It also highlights future research directions in bearing current suppression, including the development of multi-phase motor systems, real-time monitoring technologies with artificial intelligence, and the use of new insulation materials for the enhancement of bearing reliability. The results obtained should guide future research and engineering practices in suppressing bearing currents to improve motor durability with high performance.